Subsurface Laboratory Methods 221 



Preparation and Mounting of Specimen 



Too much emphasis cannot be placed on the selection of the sample 

 for analysis. Because only a very small fraction of the sample placed 

 in the camera is actually exposed to the X-ray beam, it is imperative that 

 all precautions be observed in the choice and preparation of the sample. 

 The sample chosen must be truly representative of the material being 

 investigated as regards composition, structure, or other characteristics for 

 which the sample is examined. 



Single Crystals 



Single-crystal patterns are seldom made when identification of the 

 material is the only objective, as the powder method is usually consider- 

 ably simpler. There may be occasions, however, when the sample is lim- 

 ited to a very small, pure, single crystal, insufficient in amount to grind 

 into powder. Under such circumstances, a single crystal ranging from 

 0.5 mm. to several hundredths of a millimeter in cross section and from 

 several millimeters to about 0.3 to 0.5 mm. in length is mounted on the 

 end of a small glass rod or wire, with one crystallographic axis approxi- 

 matelly parallel to the axis of the rod so that it can be mounted and ad- 

 justed in the goniometer head of the single-crystal camera to turn about 

 this axis. Patterns are recorded successively with alternate rotation about 

 the three crystallographic axes according to procedures found in standard 

 texts. ^^ From these patterns unit-cell calculations are made. Under adverse 

 conditions it may be impossible to obtain patterns about all the crystallo- 

 graphic axes, whereupon it may be necessary to calculate the dimensions 

 of the entire cell from a single rotation pattern by means of the reciprocal 

 lattice. A discussion of this concept is beyond the scope of this section, 

 but it may be found in text books on X-ray-diffraction techniques. ^^ ^° ^^ 



Powders 



For powder patterns it is usually recommended that several milli- 

 grams of representative material be crushed and ground in an agate (or 

 mullite) mortar until the entire specimen will pass a 200-mesh silk bolting 

 cloth or screen. The writer has observed that if the sample is turned or 

 oscillated during exposure to the X-rays it will be sufficient to grind the 

 sample until high lights from individual particles are no longer observed 

 when the powder is examined in a bright light. If the material does not 

 grind readily, it may be filed with a clean single-cut fine-tooth file, using 

 no more pressure than is absolutely essential. If the specimen must be 

 preserved in its original form, the specimen can be mounted in a suitable 

 rotating or oscillating device in such a manner that the sample-to-film dis- 

 tance remains constant. 



" Buerger, M. J., X-ray Crystallography, New York, John Wiley and Sons, Inc., 1942. 

 " Clark, G. L., op. ci.. 

 ^ODavey, W. P., op. cit. 

 2^Bunn, C. W., op. cit. 



